The present invention relates generally to polymer foam processing, and more particularly to formation of microcellular and other polymeric foams with extremely low levels of blowing agent.
Polymeric foams include a plurality of voids, also called cells, in a polymer matrix. By replacing solid plastic with voids, polymeric foams use less raw material than solid plastics for a given volume. Thus, by using polymeric foams instead of solid plastics, material costs can be reduced in many applications.
Microcellular foams have smaller cell sizes and higher cell densities than conventional polymeric foams. Several patents and patent publications describe aspects of microcellular materials and microcellular processes.
U.S. Pat. No. 4,473,665 (Martini-Vvedensky, et al.; Sep. 25, 1984) describes a process for making foamed polymer having cells less than about 100 microns in diameter. In the technique of Martini-Vvedensky, et al., a material precursor is saturated with a blowing agent, the material is placed under high pressure, and the pressure is rapidly dropped to nucleate the blowing agent and to allow the formation of cells. The material then is frozen rapidly to maintain a desired distribution of microcells.
U.S. Pat. No. 5,158,986 (Cha, et al.; Oct. 27, 1992) describes formation of microcellular polymeric material using a supercritical fluid as a blowing agent. In a batch process of Cha, et al., a plastic article is submerged at pressure in supercritical fluid for a period of time, and then quickly returned to ambient conditions creating a solubility change and nucleation. In a continuous process, a polymeric sheet is extruded, which can be run through rollers in a container of supercritical fluid at high pressure, and then exposed quickly to ambient conditions. In another continuous process, a supercritical fluid-saturated molten polymeric stream is established. The polymeric stream is rapidly heated, and the resulting thermodynamic instability (solubility change) creates sites of nucleation, while the system is maintained under pressure preventing significant growth of cells. The material then is injected into a mold cavity where pressure is reduced and cells are allowed to grow.
U.S. Pat. No. 5,866,053 (Park, et al.; Feb. 2, 1999) describes a continuous process for forming microcellular foam. The pressure on a single-phase solution of blowing agent and polymer is rapidly dropped to nucleate the material. The nucleation rate is high enough to form a microcellular structure in the final product.
International patent publication no. WO 98/08667 (Burnham et al.) provides methods and systems for producing microcellular material, and microcellular articles. In one method of Burnham et al. a fluid, single phase solution of a precursor of foamed polymeric material and a blowing agent is continuously nucleated by dividing the stream into separate portions and separately nucleating each of the separate portions. The divided streams can be recombined. The recombined stream may be shaped into a desired form, for example by a shaping die. Burnham et al. also describes a variety of dies, nucleators, and other arrangements for making thin articles, thick articles, extruding microcellular material onto wire, etc. In some of the methods, pressure drop rate is an important feature and techniques to control this and other parameters are described.
Foam processes, in some cases, incorporate nucleating agents, some of which are inorganic solid particles, into the polymer melt during processing. These agents can be of a variety of compositions, such as talc and calcium carbonate, and are incorporated into the polymer melt typically to promote cell nucleation. The dispersion of nucleating agents within the polymer mixture is often times critical in forming a uniform cell structure.
Blowing agents typically are introduced into polymeric material to make polymer foams in one of two ways. According to one technique, a chemical blowing agent is mixed with a polymer. The chemical blowing agent undergoes a chemical reaction in the polymeric material, typically under conditions in which the polymer is molten, causing formation of a gas. Chemical blowing agents generally are low molecular weight organic compounds that decompose at a particular temperature and release a gas such as nitrogen, carbon dioxide, or carbon monoxide. According to another technique a physical blowing agent, i.e., a fluid that is a gas under ambient conditions, is injected into a molten polymeric stream to form a mixture. The mixture is subjected to a pressure drop, causing the blowing agent to expand and form bubbles (cells) in the polymer.
International patent publication no. WO 98/08667 of Burnham, et al., published Mar. 5, 1998, employing supercritical blowing agents present at a variety of levels; International patent publication no. WO 98/31521 of Pierick, et al., published Jul. 23, 1998 describes injection molding of microcellular material, and International patent publication no. WO 99/62554 of Anderson, et al., published Jul. 1, 1999 describes microcellular extrusion/blow molding processes and articles, each reference describing the use of supercritical blowing agents at a variety of levels.
It is generally accepted in the field that to create enough nucleation sites to form microcellular foams, one must use a combination of sufficient blowing agent to create a driving force for nucleation, and a high enough pressure drop rate to prevent cell growth from dominating the nucleation event. As blowing agent levels are lowered, the driving force for nucleation decreases. Yet, while higher blowing agent levels can lead to smaller cells (a generally desirable result in the field of microcellular foams), according to conventional thought higher blowing agent levels also can cause cell interconnection (which by definition increases cell size and can compromise structural and other material properties) and less-than-optimal surface properties (compromised surface properties at higher gas levels can result from the natural tendency of the blowing agent to diffuse out of the material).
That is, it is generally accepted that there is a trade off between small cell size and optimal material properties as blowing agent levels in microcellular polymeric material are altered. It is one object of the invention to obviate this tradeoff.
The present invention provides techniques for forming microcellular polymeric articles using low levels of blowing agent. In one embodiment the invention provides a method for forming a microcellular article that involves conveying polymeric material in a downstream direction in a polymer processing apparatus. A blowing agent is introduced into the polymeric material and a mixture of the polymeric material and blowing agent is created. The blowing agent is selected among those that are gases at ambient conditions, and is added in an amount of less than about 0.08% by weight in the mixture.
Other embodiments involve significantly lower levels of blowing agent. Preferably a physical blowing agent is used, i.e. not a blowing agent formed via chemical reaction.
In preferred embodiments, a single-phase solution of blowing agent and polymeric material is created, and is nucleated by being subjected to conditions of solubility change via a rapid pressure drop. Blowing agent can be intimately mixed into the polymeric material by being introduced through many orifices of an extrusion barrel.
Microcellular material having cells of a variety of size can be formed, including various small cells. Product of a variety of weight reduction (void volumes) can be formed.
Articles can be produced by continuously extruding microcellular polymeric material, by injection molding the material, blow-molding, etc. Articles can be thermoformed after formation.
Invention also includes articles having particularly smooth surfaces, and techniques for producing these articles.
Other advantages, novel features, and objects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.